The invention relates to an electrochemical system, which comprises at least one endplate, one terminal bipolar plate as well as at least one sealing device arranged between the endplate and the terminal bipolar plate.
Known electrochemical systems of the kind mentioned are for instance fuel cell systems or electrochemical compressor systems, in particular electrolyzers. Known electrolyzers are for instance designed in such a way that upon application of an electrical potential, in addition to the production of hydrogen and oxygen from water, these gases area simultaneously compressed to higher pressure. In addition to this, electrochemical compressor systems such as electrochemical hydrogen compressors are known which are supplied with gaseous molecular hydrogen and in which this hydrogen is electrochemically compressed by the application of an electrical potential. This kind of electrochemical compression is particularly suited for small amounts of hydrogen to be compressed, since a mechanical compression of the hydrogen in this situation would be considerably more elaborate.
Further electrochemical systems are known which comprise a stack of electrochemical cells, which each are separated by bipolar plates. Such bipolar plates may for instance serve for the electrical contacting of the electrodes of the individual electrochemical cells (such as fuel cells) and/or the electrical connection of neighboring cells in case of a serial connection of the cells. The bipolar plates may further comprise a channel structure or form a channel structure, which is provided for the supply of the cells with one or several media (e.g. hydrogen, air and coolant) and/or the removal of reaction products and/or cooling media. These media can be fuels (e.g. hydrogen or methanol) or reaction gases (e.g. air or oxygen) or coolant. Such a channel structure is usually arranged in an electrochemically active area of the cell, thereby forming the flow field of the bipolar plate. It is sometimes also referred to as gas distribution structure. Further, these bipolar plates can be designed for the transport of the heat produced during the transformation of electrical or chemical energy in the electrochemical cell as well as for the sealing of the different reaction media or coolant channels against each other and towards the outside.
The bipolar plates may for instance comprise openings, through which the media and/or the reaction products and/or coolant are guided to or from the electrochemical cells arranged between the bipolar plates of the stack adjoining each other. These electrochemical cells may for instance each comprise one or several membrane-electrode assemblies, usually abbreviated as MEA. The MEA may comprise at least one electrolyte membrane and at least one electrode, preferably one electrode on both its sides. Further, two gas diffusion layers, abbreviated as GDLs, are situated adjacent to the MEA; these GDLs are usually oriented towards the bipolar plates and realized as metallic or carbon fleece or carbon paper.
Bipolar plates are usually constructed from two independent plates which are connected to each other at least in sections. In the following these independent plates are referred to as half plates.
In general, the stack comprising the bipolar plates and the electrochemical cells is terminated at both its ends by an endplate. At least one of the endplates typically comprises one or several ports. The pipes for the supply of the media and/or the removal of the reaction products may be connected to these ports. In addition, at least one of the endplates usually comprises electrical connections, via which the cell stack can be electrically connected to an electrical load or a source of electrical voltage. The bipolar plate of the stack situated closest or adjacent to such an endplate is also referred to as terminal bipolar plate.
No medium is guided between the terminal bipolar plate and the endplate which goes along with no electrochemical reaction taking place in this interspace. As a consequence, no proton-conductive membrane is arranged between the terminal bipolar plate and the endplate. Rather, the current collector is arranged in this interspace. It is therefore not surprising that in most electrochemical stacks, the terminal bipolar plate(s) are designed different from the repeating bipolar plates in the stack. This is especially true with respect to the supply of media from the ports to the surface of the terminal bipolar plate facing the endplate. As no medium has to be provided to this interspace, no passage is provided on this surface. In contrast, each nonterminal bipolar plate comprises passage which allow for the passage of media from the ports to the corresponding surfaces of the bipolar plate. The same is true for the other surface of the terminal bipolar plate. Thus, both half plates of “ordinary” bipolar plates comprise passages from the ports to the flow field, while only one half plate of the terminal bipolar plate, namely the one facing the bipolar plate stack, comprises a passage from a port to the flow field.
A sealing device is typically arranged between the terminal bipolar plate and the endplate. It serves for the sealing of the system to the outside and/or the sealing of various pipes or sections of the electrochemical system against each other. The sealing between the terminal bipolar plate and the endplate in known systems is for instance realized by metallic beads, which are preferably one-piece with the terminal bipolar plate and screen-printed micro-seal coating applied to the bead. However, this screen-printed material tends to stick in particular to the mechanically treated, at least slightly rough plastic surfaces of the endplates. It is also possible that the sealing device is removed from the metallic bead or destroyed if the terminal bipolar plate and the end plate, which in general are produced from different material and therefore both have different coefficients of thermal expansion, are moving relative to each other, in particular laterally, thus in the direction orthogonal to the stack direction, as a consequence of changes in temperature of the electrochemical system. The bipolar plates and therefore also the terminal bipolar plate are formed from metal, e.g. stainless steel, while the endplate is produced from plastics or essentially from plastic. In some applications, the sealing device has to reliably function in the same manner in a temperature range between a minimum temperature of e.g. −40° C. and a maximum temperature of e.g. +100° C. Such temperature changes are especially encountered between the operation start of the fuel cell system being at environmental temperature especially the cold start at negative ambient temperatures in winter and transition to the maximum operation temperature of the stack. The consequences of the removal and sticking of the coating is particularly obvious when the stack is demounted, as the coating due to the prior removal is torn away from the terminal bipolar plate.
In order to avoid or at least reduce this relative shift during temperature change, the endplate might also be constructed from metal. However, this both increases the production cost and the weight of the system, which is not desired for many applications. On the other hand, the sealing of the interface between the endplate and the terminal bipolar plate with rubber gaskets, e.g. O-ring seals or so called floppy gaskets, which are partly seated in at least one of the plates, can lead to difficulties with the adjustment of the height and force of the sealing system due to the strong settling of such gaskets.